The present invention relates to a method for producing a nanocomposite magnet as a composite of crystallites of a Fe boride such as Fe3B and crystallites of a Rxe2x80x94Fexe2x80x94B compound such as R2Fe14B. The nanocomposite magnet is suitably used for motors, actuators, Magrolls, and the like.
An Fe3B/Nd2Fe14B nanocomposite magnet is a permanent magnet where crystallites of a soft magnetic Fe boride such as Fe3B and crystallites of a hard magnetic R2Fe14B compound such as Nd2Fe14B are uniformly distributed in the same metal structure and magnetically coupled to each other as a result of exchange interactions, therebetween. Each of these crystallites is of a size on the order of several nanometers, and the magnet has a structure where these two types of crystalline phases are combined (nanocomposite structure). Thus, the magnet of this type is called a xe2x80x9cnanocomposite magnetxe2x80x9d.
Although the nanocomposite magnet contains the soft magnetic crystallites, it exhibits excellent magnetic properties by the magnetic coupling of the soft magnetic crystallites with the hard magnetic crystallites. In addition, since the soft magnetic crystallites do not include any rare-earth element such as neodymium (Nd) that is expensive, the total volume fraction (concentration) of rare-earth elements in the magnet is small. This reduces the production cost and thus is suitable for stable supply of the magnet.
The nanocomposite magnet of this type is produced by quenching a molten material alloy to form a rapidly solidified alloy including an amorphous phase and then heat-treating the solidified alloy to allow crystallites to be generated in the solidified alloy.
In general, the rapidly solidified alloy is prepared by a melt-spinning technique such as a single roller method or a liquid quenching technique such as a strip-cast method. According to the liquid quenching technique, a melt of a material alloy is cast to the outer circumference of a rotating chill roller, to come into contact with the roller for just a short period of time, thereby quenching and solidifying the material alloy. In this method, the cooling rate is controllable by adjusting the surface velocity of the rotating chill roller and the amount of the molten metal supplied to the chill roll.
The alloy that has been solidified and detached from the chill roller is in the shape of a ribbon (or strip) elongated along the circumference of the roller. The ribbon of alloy gets crushed into flakes by a crusher and then pulverized into finer powder particles by a mechanical grinder.
Thereafter, the powder particles are heat-treated to crystallize. As a result, crystallites of a soft magnetic Fe boride and crystallites of a hard magnetic R2Fe14B compound are grown in the same metal structure and magnetically coupled together through the exchange interactions.
The type of the metal structure resulting from the heat treatment in the production process plays a key role in improving the properties of the nanocomposite magnet as a final product. The conventional heat treatment process, however, has various drawbacks in view of the controllability and reproducibility thereof. Specifically, since a large quantity of heat is generated in a short time during the crystallization of the amorphous material alloy, it is difficult for a heat treatment apparatus to control the temperature of the processed alloy. If a great amount of material alloy powder were subjected to the heat treatment at a time, in particular, the temperature of the alloy powder would almost always be out of control. Thus, according to the conventional technique, the heat treatment should be performed on just a small amount of material powder at a time and the resultant processing rate (i.e., the amount of powder procesable per unit time) is far from being satisfactory. Such a low processing rate constitutes a serious obstacle to mass-production of magnet powder.
An object of the present invention is providing a method for preparing efficiently and reproducibly powder of a nanocomposite magnet as a composite of crystallites of a soft magnetic Fe boride and crystallites of a hard magnetic R2Fe14B compound that are distributed uniformly in the same metal structure and magnetically coupled together through the exchange interactions.
The method for preparing nanocomposite magnet powder of the present invention includes the steps of: preparing material alloy powder for a nanocomposite magnet, the powder being represented by a general formula Fe100xe2x88x92xxe2x88x92yRxBy, Fe100xe2x88x92xxe2x88x92yxe2x88x92zRxByCoz, Fe100xe2x88x92xxe2x88x92yxe2x88x92uRxByMu or Fe100xe2x88x92xxe2x88x92yxe2x88x92zxe2x88x92uRxByCozMu, where R is a rare-earth element; 78-100 atomic percent of R is Pr and/or Nd, while 0 to 22 atomic percent of R is another lanthanoid and/or Y; M is at least one element selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Ga, Zr, Nb, Mo, Hf, Ta, W, Pt, Pb, Au and Ag; the molar fractions x, y, z and u meet the inequalities of 2xe2x89xa6xxe2x89xa66, 16xe2x89xa6yxe2x89xa620, 0.2xe2x89xa6zxe2x89xa67 and 0.01xe2x89xa6uxe2x89xa67, respectively, the powder including a metastable phase and an amorphous structure existing in a metal structure; and heat-treating the material alloy powder for the nanocomposite magnet, thereby crystallizing Fe3B and Fexe2x80x94Rxe2x80x94B compounds from the amorphous structure, wherein an integral value of the difference between a temperature-time curve represented by the temperature of the material alloy powder for the nanocomposite magnet as a function of the heat treatment time in the step of heat-treating the material alloy powder for the nanocomposite magnet and a reference temperature-time curve is in a range from 10xc2x0 C.xc2x7sec to 10,000xc2x0 C.xc2x7sec, the reference temperature-time curve being obtained when heat treatment similar to the step of heat-treating the material alloy powder for the nanocomposite magnet is performed for an equivalent amount of alloy that has the same composition as the material alloy for the nanocomposite magnet but does not include the amorphous structure.
Preferably, in the step of heat-treating the material alloy powder for the nanocomposite magnet, the temperature of the material alloy for the nanocomposite magnet rises at a temperature rise rate in a range from 1xc2x0 C./min to 100xc2x0 C./sec, from a temperature T1, at which a difference starts to arise between the temperature-time curve for the material alloy powder for the nanocomposite magnet and the reference temperature-time curve, to a highest temperature T2 exhibited by the material alloy powder for the nanocomposite magnet. It should be noted that this temperature-time curve difference is due to heat generation by crystallization of the amorphous structure.
In the step of heat-treating the material alloy powder for the nanocomposite magnet, preferably the highest temperature T2 exhibited by the material alloy powder for the nanocomposite magnet is in a range from 620xc2x0 C. to 800xc2x0 C.
Preferably, the step of preparing the material alloy powder for the nanocomposite magnet includes the steps of: forming a melt of the material alloy; rapidly solidifying the melt; crushing the rapidly solidified material alloy; and pulverizing the material alloy, wherein during the step of rapidly solidifying the melt a cooling rate of the alloy is defined within the range from 5xc3x97103 K/s to 5xc3x97106 K/s, and the temperature of the quenched alloy is lower by 400xc2x0 C. to 800xc2x0 C. than the temperature Tm of the molten alloy yet to be quenched. Further, the step of molding the powder includes producing a bonded magnet out of the heat-treated material alloy powder.
The motor of the present invention includes a nanocomposite magnet produced by any of the methods for producing a nanocomposite magnet described above.